1. Field of the Invention
The present invention relates to the process of displaying characters on a video monitor, and more particularly to techniques for antialiasing fonts for display on an interlaced display, such as a television monitor.
2. Related Art
The standard television format in the United States is NTSC (National Television System Committee), which is encoded in an interlaced form. In NTSC, each frame consists of two interlaced fields which are alternately displayed on a television monitor. One field consists of all of the odd lines of a frame. The other field consists of all the even lines. One artifact of this arrangement is that fine details with high contrast, such as thin white lines on a black background, may be contained only within one of these fields. This causes fine details to flicker on television display, because a field which contains a fine detail is refreshed at half the frequency of the entire frame, and is alternated with the alternate field, which does not contain the fine detail. This causes detailed images such as text to be very unreadable on an interlaced television display.
Antialiasing techniques have been developed to smooth out the jagged edges which result from scan converting the edges of an object for bit-mapped display. These jagged edges are an instance of a phenomenon known as "aliasing." Techniques which are used to reduce or eliminate aliasing are referred to as "antialiasing", and images produced using these techniques are said to be "antialiased". Antialiasing techniques operate by blurring the jagged edges so that they appear to the human eye to be more smooth. This blurring is accomplished by assigning intermediate shading values to the pixels surrounding the edge of an object, wherein these intermediate color values are a mixture of the color of the object and a background color against which the object appears.
Standard antialiasing techniques such as filtering effectively reduce the jagged edges in a standard non-interlaced display such as a computer monitor. However, standard anti-aliasing techniques do not adequately reduce the flickering which occurs near edges of objects in an interlaced display. This flickering makes it especially hard to read characters on an interlaced display; especially characters which are outputted using intricate fonts.
What is needed is a technique for generating antialiasing fonts for display on an interlaced monitor which more effectively reduces the flickering of character features, thereby enhancing the readability of characters on interlaced displays.